In a typical application of a chain tensioner, a transmission chain is engaged with, and driven by, an engine crankshaft sprocket, and is in driving relationship with one or more sprockets provided on engine valve-operating camshafts. A part of the chain, usually the part that moves from the crankshaft sprocket toward a camshaft sprocket, is in sliding relationship with shoe of a movable tensioner lever, which is biased by a tensioner in a direction to maintain tension in the chain.
As shown in FIG. 8, in a conventional engine timing drive, a timing chain C is engaged with a driving sprocket S1, mounted on a crankshaft, and with a pair of driven sprockets, S2 and S3, mounted on valve-operating camshafts. The timing drive includes a stationary chain guide G2, mounted on bolts B1 and B2, and a movable chain guide G1, pivoted on a bolt B, and pressed against the chain by the plunger of a tensioner 500 in order to maintain tension in the chain.
An example of a known chain tensioner is described in United States Patent Application publication No. 2002/0052259, published May 2, 2002. As shown in FIG. 7, the known chain tensioner 500 comprises a tensioner housing 510 having a cylindrical plunger-accommodating hole 511 in which a plunger slides. The plunger has a cylindrical exterior surface through part of its length, which fits closely the cylindrical interior surface of the plunger-accommodating hole. The plunger protrudes through an opening at one end of the plunger-accommodating hole. A coil spring 540 extends into a hole 521 in the plunger 520, and is compressed between the bottom of the plunger-accommodating hole 511 and the end of hole 521 in the plunger. The coil spring serves as a biasing means, continuously urging the plunger 520 in its protruding direction.
Oil is supplied under pressure to the plunger-accommodating hole 511 through an oil inflow channel 512 provided in the housing, and fills a high pressure oil chamber 513 formed by the plunger-accommodating hole 511 and the plunger 520. As illustrated in FIG. 5, slow leakage of oil from the high pressure oil chamber 513 through a small gap between the exterior of the cylindrical part of the plunger and the cylindrical internal wall of the plunger-accommodating hole 511 causes damping of the reciprocating movement of the plunger 520. The damping property of the tensioner depends on factors such as oil viscosity and the dimensions of the gap between the plunger and the cylindrical wall of the plunger-accommodating hole.
As shown in FIG. 7, the plunger 520 is also provided with a relief valve 522, which allows oil to flow out the high-pressure oil chamber 513 when the pressure within the chamber becomes excessive, as disclosed in United States Patent Application publication No. 2002/0052259.
The plunger damping effect is important for the suppression of fluctuations in chain tension and sinusoidal vibrations of the chain. However, the effectiveness of the damping action is highly dependent, not only on the engine, but on changes in the operating conditions of a given engine. Accordingly, adjustments must be made for optimum damping. However, the gap between the plunger 520 and the plunger-accommodating hole 511 is determined when the tensioner is designed, with the primary objective of ensuring that the plunger reciprocates smoothly and uniformly. Hence it is difficult to make different adjustments for optimum damping of tensioners individually.
Although it is possible to control damping by providing a V-shaped groove 523 on the side the plunger 520 as shown in FIG. 6, and to control damping property by varying the width and depth of the groove, it is necessary to determine the appropriate groove dimensions for each engine in which the tensioner is to be used, and, once the damping property has been set, it is impractical to change it to accommodate different operating conditions.
Accordingly, the damping properties of a tensioner have been a compromise, arrived at by determining a setting that provides adequate, but not optimum, damping under a variety of operating conditions. In addition, it has not been practical heretofore to correct the damping properties of a tensioner when they depart from designed damping properties as a result of machining errors.
Another problem with the known chain tensioner shown in FIG. 7 is that the relief valve 522, which is provided as a countermeasure against excessive pressure in the oil chamber 513, cannot be used to adjust the tensioner's damping properties.